Magor Katharine E, Shum Benny P, Parham Peter
Department of Structural Biology, Stanford University, Stanford, CA 94305, USA.
J Immunol. 2004 Mar 15;172(6):3635-43. doi: 10.4049/jimmunol.172.6.3635.
Beta2-microglobulin (beta2m) associates with MHC and related class I H chains to form cell surface glycoproteins that mediate a variety of functions in defense. In humans, monomorphism of a single beta2m gene contrasts with the diversity and polymorphism of the class I H chain genes, and a similar picture was seen in almost all other species examined. In this regard, rainbow trout (Oncorhynchus mykiss) appeared unusual: trout beta2m genes gave a complicated and polymorphic pattern in Southern blots, and a minimum of 10 different mRNA encoding two distinct types of beta2m were expressed by a single fish. Characterization of genomic clones from the same fish now shows that the rainbow trout beta2m locus consists of two expressed genes and one partial gene that are closely linked. Four copies of the locus were identified and allelic variants of each gene defined, largely through comparison of the noncoding regions. A dramatic variation in the lengths of introns is caused by variable repetitive elements and accounts for the complex pattern seen in Southern blots. By comparison to noncoding sequences, the coding regions are conserved but the three loci differ within a cluster of codons that encode residues of beta2m that do not interact with class I H chains. Additional diversity in the trout beta2m genes appears to be due to somatic mutation that might be facilitated by the abundance of repetitive DNA elements within the 12 beta2m genes of an individual rainbow trout.
β2-微球蛋白（β2m）与主要组织相容性复合体（MHC）及相关的I类重链结合，形成细胞表面糖蛋白，介导多种防御功能。在人类中，单个β2m基因的单态性与I类重链基因的多样性和多态性形成对比，在几乎所有其他被检测的物种中也观察到类似情况。在这方面，虹鳟（Oncorhynchus mykiss）显得不同寻常：虹鳟β2m基因在Southern印迹中呈现出复杂的多态性模式，一条鱼至少表达10种不同的编码两种不同类型β2m的mRNA。对同一条鱼的基因组克隆进行表征，结果表明虹鳟β2m基因座由两个表达基因和一个部分基因紧密连锁组成。通过对非编码区的比较，鉴定出该基因座的四个拷贝，并确定了每个基因的等位基因变体。内含子长度的显著差异是由可变重复元件引起的，这解释了Southern印迹中看到的复杂模式。与非编码序列相比，编码区是保守的，但这三个基因座在编码不与I类重链相互作用的β2m残基的密码子簇内存在差异。虹鳟β2m基因的额外多样性似乎是由于体细胞突变，个体虹鳟的12个β2m基因中丰富的重复DNA元件可能促进了这种突变。
